Two new primitives: index_offset() + maildir_offset().

  These return the current offset into the message-list, and
 maildir-list respectively.

  Useful for iterating over messages/folders from the current
 position.

  Closes #113.
Merge remote-tracking branch 'upstream/master' into mutt_index_jumpstyle
key-bindings and use on_mode_change

The logic when switching form maildir to index is now called from the
on_mode_change hook. I also added keybindings to move to the next unread
mail in index or message mode.